Should I stop sending to Outlook/Hotmail during IP warming due to blocked bounces?
Michael Ko
Co-founder & CEO, Suped
Published 27 Apr 2025
Updated 18 Aug 2025
7 min read
Warming up a new IP address is a delicate process, and it's common to encounter unexpected hurdles. One of the most frustrating scenarios is seeing an increase in blocked bounces, especially from major mailbox providers like Outlook or Hotmail, even when your Microsoft SNDS (Sender Network Data Services) status appears to be green. This situation often leads to a crucial question: should I stop sending to these domains altogether during the warming period?
The short answer is usually no. Halting your sends entirely can be more detrimental than continuing with adjustments. IP warming relies on consistent, gradual sending to build a positive reputation with internet service providers (ISPs). Abruptly stopping can reset the progress you've made, making future deliverability even harder to achieve. The goal is to show consistent, wanted mail.
Instead of a full stop, a more nuanced approach is required. This involves understanding why these blocked bounces are occurring and implementing strategic adjustments to your sending practices. It is a balancing act between maintaining sending consistency and addressing reputation signals.
Understanding the blocked bounces
When you encounter blocked bounces from Outlook or Hotmail during IP warming, the first step is to thoroughly investigate the bounce messages themselves. While your overall SNDS status might be green, individual bounce codes often provide specific reasons for the block. These reasons can range from rate limiting to content-based filtering or even a temporary IP block (also known as a blacklist). Without knowing the precise error, it's difficult to formulate an effective response.
Microsoft is known for its strict filtering policies, especially for new or warming IP addresses. They closely monitor sender behavior, and even a slight deviation from expected patterns during the ramp-up can trigger blocks. Your SNDS data is a valuable tool here, but it may not always reflect immediate, tactical blocks. Sometimes, the green status indicates a good general reputation, but specific thresholds for new IPs are still being triggered.
If you don't have direct access to the full bounce messages, work with your Email Service Provider (ESP) or look into your deliverability reporting tools. They should be able to provide the exact error codes. These codes are critical for diagnosing the problem, as different codes suggest different solutions, from throttling volume to checking your content or even your authentication setup like SPF, DKIM, and DMARC. Understanding these nuances is crucial for navigating Microsoft's email ecosystem.
Typical Outlook Bounce Codes
421 RP-001: Your IP has exceeded the rate limit. This is common during IP warming if volume increases too quickly.
550 SC-001: IP reputation issue. Your IP might be blocked (blacklisted) due to suspicious activity, high complaints, or spam trap hits.
550 OU-001: User complaints have exceeded acceptable thresholds. This indicates a quality issue with your list or content.
550 S3150 / S3140: Indicates a block due to suspected spamming or compromised IP. Requires immediate action and often delisting requests.
Adjusting your sending strategy
My advice is not to completely stop sending to Outlook/Hotmail domains. A complete halt can be interpreted by ISPs as a cessation of legitimate sending, potentially leading to a quicker decay of any nascent IP reputation you've built. Instead, the focus should be on adjusting your sending volume and strategy for those specific domains. You might need to temporarily reduce the volume you send to Microsoft properties back to a level where you weren't experiencing bounces, even if you continue to ramp up for other ISPs.
You can also implement a targeted approach, using a regex filter or segmentation in your sending platform to specifically control volume to Microsoft addresses. This allows you to maintain consistent (though adjusted) sending activity to these domains while allowing your overall IP warming plan for other mailbox providers to continue on schedule. Remember that sending caps are strict during this period.
Another crucial aspect is to ensure you're sending to your most engaged subscribers on Outlook/Hotmail. These are the users most likely to open your emails, click links, and, importantly, move your emails from the spam folder to the inbox if they land there. This positive engagement sends strong signals to Microsoft about the legitimacy and desirability of your mail, helping to build your IP's reputation and potentially leading to the lifting of blocks.
The risk of stopping completely
Loss of progress: Your IP's warming journey might reset, requiring you to start from a much lower volume.
Reputation decay: Inactivity can cause your new IP's reputation to stagnate or decline.
Delayed ramp-up: Your overall migration timeline will be significantly extended.
The benefit of adjusting volume
Consistent signaling: Shows ISPs (like Microsoft) you are a regular sender, even at a reduced rate.
Reputation building: Allows for gradual reputation improvement with targeted engagement.
Faster resolution: You can still troubleshoot while maintaining some sending activity.
Proactive reputation management
Even if you encounter blocked bounces, it's crucial to proactively work on improving your sender reputation with Microsoft. This isn't a passive waiting game. A key part of this involves addressing any issues identified by the bounce messages, such as content, authentication, or list quality. Engaging with Microsoft's support channels is also important.
Clean your email list thoroughly to remove any inactive or invalid addresses, which can lead to hard bounces and spam trap hits. Also, focus on maintaining consistent sending patterns as part of your IP warming. Avoid large, sudden spikes in volume, even if you feel the block has been lifted. Gradual increases are always better. For specific guidance on dealing with blocklists, there are comprehensive guides available.
Monitoring your deliverability beyond just SNDS is also crucial. Pay attention to inbox placement rates, complaint rates, and overall engagement for your sends to Microsoft properties. This holistic view helps you spot subtle changes in reputation that might not immediately trigger a red flag in SNDS but could still impact your long-term deliverability. Microsoft emphasizes sender IP reputation in their filtering decisions.
Example of a detailed bounce message (SMTP 550)text
550 5.7.1 SCL is -1, PCL is 8, InfoD is 10, HELO is [192.0.2.1], SPF is None, FQDN is [mail.example.com], Policy is SFP_BLOCK
Maintaining momentum with other ISPs
While you're adjusting your strategy for Microsoft, continue with the remaining IP warming plan for other inboxes. Different ISPs have different warming requirements and thresholds. What triggers a temporary block (or blacklist) at Outlook might not affect Gmail or Yahoo. Maintaining forward momentum with other providers is important for your overall IP reputation and the successful completion of your warming process.
The key is to segment your sending audiences appropriately. If you know Microsoft is being particularly sensitive, throttle or pause only the sends to those domains, rather than your entire mailing list. This targeted approach allows you to address the specific issue without jeopardizing your broader warming efforts. Consider how to target mailbox providers effectively during a warm-up.
Always prioritize sending to your most engaged subscribers first, regardless of the domain. These are the users who will help establish a positive sending history. As your reputation improves, you can gradually expand to less engaged segments. This strategy minimizes the risk of triggering further blocks and helps you achieve consistent deliverability across all major mailbox providers, even when one is proving to be a challenge.
Final thoughts on IP warming challenges
Navigating IP warming, especially when you hit a snag like blocked bounces from Outlook/Hotmail, requires patience and a methodical approach. The worst thing you can do is panic and stop sending entirely. Instead, gather data, adjust your sending volume strategically, prioritize engaged recipients, and continue building your reputation across other ISPs. This ensures you maintain the warming process while addressing specific issues head-on.
Remember that consistent, desired mail is the ultimate goal. By meticulously following these steps, you can overcome temporary deliverability challenges and successfully complete your IP warming, setting a strong foundation for your email program.
Views from the trenches
Best practices
Maintain consistent, even if reduced, sending volume to Microsoft properties to avoid reputation decay during warming.
Segment your audience to send only to the most engaged Outlook/Hotmail users initially to generate positive signals.
Continuously monitor all bounce messages and error codes to understand specific reasons for blocks.
Ensure all email authentication (SPF, DKIM, DMARC) is correctly configured and aligned.
Keep sending to other mailbox providers as per your original IP warming schedule if they are not experiencing issues.
Common pitfalls
Completely stopping email sends to an ISP, which can reset or halt your IP warming progress.
Ignoring bounce messages and not identifying the specific error codes provided by Microsoft.
Attempting to ramp up volume too quickly after encountering initial blocks or throttling.
Sending to unengaged or old email addresses on Microsoft domains during the sensitive warming period.
Failing to engage with Microsoft's sender support or SNDS portal for specific block (blacklist) issues.
Expert tips
If you get blocked by Microsoft, check your bounce messages for specific codes like RP-001 or SC-001, which indicate rate limiting or reputation issues respectively.
Leverage email engagement data to prioritize sends to active subscribers during IP warming, especially with challenging ISPs.
Consider a phased approach where you target smaller, highly engaged segments first, then gradually broaden your audience as reputation improves.
Regularly review your Microsoft SNDS data beyond just the green status, looking for any subtle increases in complaint rates or yellow filters.
Don't solely rely on automated warming tools; actively monitor and adjust your sending based on real-time deliverability feedback.
Marketer view
Marketer from Email Geeks says: You should not stop sending entirely, but it is best to stop adding new volume and potentially take the volume back to where it was just before the problem began.
2024-03-12 - Email Geeks
Marketer view
Marketer from Email Geeks says: It is critical to know the exact bounce message to diagnose the issue correctly.
Outlook or 
Hotmail, even when your Microsoft SNDS (Sender Network Data Services) status appears to be green. This situation often leads to a crucial question: should I stop sending to these domains altogether during the warming period?
Microsoft addresses. This allows you to maintain consistent (though adjusted) sending activity to these domains while allowing your overall IP warming plan for other mailbox providers to continue on schedule. Remember that sending caps are strict during this period.
Gmail or 
Yahoo. Maintaining forward momentum with other providers is important for your overall IP reputation and the successful completion of your warming process.
